Conventional pollution treatment technologies, such as carbon adsorption, air stripping and bio-oxidation, are effective in removing many organic pollutants from water streams, but they exhibit a major drawback. Both carbon adsorption and air stripping merely transfer the organic pollutants from one medium to the other. Biological treatment, on the other hand, is relatively slow and inefficient in removing many man-made organic pollutants such as chlorinated hydrocarbons. An efficient technology is required, which will not only remove the organic pollutants from the water streams, but also destroy them.
Among advanced photo-oxidation processes, the TiO.sub.2 photocatalyzed removal of organic pollutants has recently begun to show considerable promise. The TiO.sub.2 process is very well documented in the literature. The illumination of TiO.sub.2 in water with light of wavelength &lt;400 nm generates excess electrons in the conduction band (e.sup.-.sub.CB) and positive holes (h.sup.+.sub.VB) in the valence band. EQU TiO.sub.2 +h.fwdarw.e.sup.-.sub.CB +h.sup.+.sub.VB ( 1)
At the surface the holes either react with adsorbed H.sub.2 O or surface OH.sup.- groups to form OH radicals. EQU h.sup.+.sub.VB +H.sub.2 O(ads.).fwdarw..sup.. OH+H.sup.+ ( 2a) EQU h.sup.+.sub.VB +OH.sup.- (sur.).fwdarw..sup.. OH (2b)
Excess electrons in the conduction band probably react with molecular oxygen to form superoxide ions EQU e.sup.-.sub.CB +O.sub.2 .fwdarw.O.sub.2.sup..- ( 3)
which can further disproportionate to form more OH radicals. EQU 20.sub.2.sup..-+ 2H.sub.2 O.fwdarw.2.OH+20H.sup.- +O.sub.2 ( 4)
The OH radicals are extremely reactive and readily attack organic molecules, eventually degrading them to CO.sub.2 and H.sub.2 O (and halide ions when the organic molecule contains halogen atoms). Thus the advantage of this technology is that it not only removes, but also destroys organic pollutants.